Basically the cPanel provides three webmail clients .

1) In Horde >> Options >> Locale and Time >> here you need to choose the desired Time zone from the drop down menu then click the Save button.

2) In RoundCube >> Personal Settings >> Preferences >> here you can select the desired Time zone from the drop down menu then click the Save button.

3) SquirrelMail >> There is no option to change the time zone. So you cannot adjust the Time zone settings in SquirrelMail.

Done..